Assignee employs a rotary hearth furnace (RHF) to recover and recycle valuable nickel, chromium and iron from steel plant wastes such as flue dust, sludge, turnings, etc. In a separate process, it also directly reduces iron oxide in the RHF.
In assignee's operations, metallic plant wastes are first pelletized with coal and then partially reduced in the RHF. The entrained carbon (from the coal) reacts with oxygen in the RHF to produce carbon monoxide which in turn reduces the nickel and iron. The resultant partially sintered pellets are then subsequently treated in an electric arc smelting furnace wherein the chromium is reduced. Ultimately, a rough intermediate 18-8 stainless steel pig is produced. The pig is recycled to the stainless steel industry for reintroduction into their furnaces as ancillary feedstock.
Briefly, an RHF is a continuous reheating furnace generally having a circular inner wall circumscribed by a spaced circular outer wall. The circular void formed therebetween includes an annular rotating hearth. In order to retain and reflect the heat generated within the furnace, the walls are relatively low so as to enable the roof to be close to the hearth. Burners may be installed in the inner and outer walls and in the roof.
Material is usually loaded onto the rotating hearth by dropping it with a conveyor or chute. After the material is carried on the hearth, it is usually removed by a discharge or conveying screw. Due to high temperatures (1300.degree.-2300.degree. F. 704.degree.-1260.degree. C.!), the screw is water cooled. See U.S. Pat. No. 3,443,931. Gases are permitted to vent through a flue located in the roof.
A conveying or discharge screw typically consists of a central shaft with a series of helical flights welded thereto. A cooling fluid is passed through the screw. U.S. Pat. No. 4,636,127 (assignee's current design) discloses a discharge screw having water cooled hollow flights.
The discharge screw conveys the reduced pellets from the hearth bed down a refractory chute and into containers. The discharge screw extends across the width of the donut shaped hearth and is connected to a motor for rotation.
The screw is mounted on a trunnion to allow for height adjustment above the hearth. In order to remove the screw from the furnace, the screw must be first disconnected from its moorings and couplings and then upwardly removed through the roof; a difficult job.
Due to the corrosive nature of the gases and materials present within the RHF, coupled with the high temperatures therein, the discharge screw is subject to frequent failure. The screw barrel and the hollow flights eventually deteriorate. Corrosion and erosion caused by high temperatures, tough particles and bad actors (sodium, sulfides, chlorides, fluorides) within the RHF inexorably chew up the screws and render them useless after about five months.
In addition, the spaces between the flights accumulate fluffy fines that tend to cake together. The fines act as a sponge which serves to collect and concentrate the corrosive gases present within the furnace.
The barrel of the discharge screw originally was fabricated from a butt-welded carbon steel tube. Service life of the tube declined as levels of contaminants (in particular chlorine) in the furnace environment increased. The surface of the barrel would corrode away until water leaks developed necessitating replacement of the entire discharge screw. Service life of the plain carbon steel barrel ranged from four to ten months.
Similar surface corrosion was also observed on the surface of the plain carbon steel discharge screw trunnions that also operate within the furnace atmosphere. As a result, each time a discharge screw was removed from service these trunnions were extensively remetallized to bring their wall thickness back to the original diameter.
Currently, flights are cast from HH alloy (20% nickel, 20% chromium) and are weld overlaid with Inconel.RTM. alloy 72 (55% nickel, 45% chromium) on both surfaces of the flight. (Inconel is a trademark of the Inco family of companies). The purpose of the overlay is to inhibit corrosion of the surface of the flight where it historically corrodes in an "hour glass" pattern along the thickness of the flight. Flights are welded to the barrel using Inconel alloy 82 filler metal. No problems have been observed in the weld area so Inconel alloy 82 continues to be the alloy of choice for welding. This design has resulted in an average service life of 61/2 months. Even with the overlay, the tip of the flight ultimately breaks off at a location approximately one to two inches (2.54-5.08 cm) up from where the flight is welded to the surface of the barrel.
As can be appreciated, frequent screw replacement necessitates frequent downtime, high maintenance and labor costs, and inefficient use of the furnace which in turn leads to higher unit costs. Clearly a longer lasting screw design is necessary.